The present invention relates to a liquid crystal display (LCD) and, more particularly, to a thin film transistor (TFT) for controlling current supply to a pixel electrode.
While TFTs are extensively used in the drive circuits of various kinds of devices as switching elements, the application of TFTs to LCDs is conspicuous. However, a TFT for use in an LCD has the following problem. Assume that a semiconductor layer included in the TFT is implemented by amorphous silicon (a-Si:H) often applied to, e.g., LCDs. Then, a photocarrier is generated in the a-Si:H layer by light output from a backlight which is a light source for display. The photocarrier migrates in regions where electric field control by a gate electrode and a drain electrode and a source electrode is weak, enhancing a light OFF current. As a result, leak occurs via a pixel electrode and degrades display quality. It is a common practice to reduce light incident to a TFT by a shield layer provided on a substrate facing a glass substrate or on the glass substrate. The shield layer, however, is not fully satisfactory because when the backlight is intense, light enters the TFT due to reflection and diffusion. This problem is particularly serious with, e.g., a monitor LCD or an on-board LCD needing a high luminance backlight.
As for the light OFF current, Japanese Patent Laid-Open Publication No. 7-147411 proposes to locally reduce the area of a low concentration diffusion layer included in a semiconductor device, thereby reducing the photocarrier. Japanese Patent Laid-Open Publication No. 7-94753 discloses a structure including a gate electrode which is provided with a greater area than a semiconductor layer so as to screen the entire semiconductor layer.
The technology taught in the above Laid-Open Publication No. 7-147411 has a problem that it reduces even the effective width of the channel of the TFT and therefore the ON current. Such a technology is not feasible for a TFT using a-Si:H, among others. Laid-Open Publication No. 7-94753 mentioned above also reduces the ON current because the contact portions between the source electrode and drain electrode and the semiconductor layer are screened; particularly, resistance increases at the contact portions when use is made of a-Si:H.
Technologies relating to the present invention are also disclosed in AMLCD ""94 and Japanese Patent Laid-Open Publication Nos. 7-114046, 7-326763, and 8-32073.
It is therefore an object of the present invention to provide a TFT capable of reducing a light OFF current while preventing a light ON current from being reduced.
In accordance with the present invention, in a TFT for an LCD and having a gate electrode, a gate insulation film, an island-like semiconductor layer, a drain electrode and a source electrode sequentially laminated on a transparent insulative substrate, a part of the semiconductor layer above the gate electrode in the direction of channel length has a smaller dimension than the other part in the direction of channel width.